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Tests were carried out on six modern houses over the course of five days by Professor Alastair Lewis of the National Centre for Atmospheric Science at the University of York.

Firstly he measured the levels of a range of so-called volatile organic compounds (VOCs) in the air such as benzene, which comes from outdoor vehicle pollution.

But the most prevalent chemical he found in the house was limonene, which is released by fragranced candles, plug-ins, air fresheners and cleaning products.

Limonene is commonly used to give a citrus smell to scented candles and cleaning products, and is considered so safe it is used to flavour food.

However, once sprayed into our homes it doesn't stay as limonene - as it reacts with other gases which occur naturally in the air.

Back in the laboratory, Prof Lewis found that when limonene is exposed to ozone - which is naturally present in the air - every two molecules of limonene could produce one molecule of formaldehyde. Formaldehyde is used in embalming and heavy industry. It is a known cause of cancer in humans, and is most closely linked with cancers of the nose and throat.

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At the very least it can cause sore throats, coughs, stinging eyes and nosebleeds.

But before you start binning those oil burners, Prof Lewis also found that certain houseplants - particularly English ivy, geraniums, lavender and ferns - were good at absorbing the formaldehyde in buildings.

He continued to analyse the air in the six houses for a further four weeks, and he found that while the levels of limonene stayed constant, formaldehyde dropped markedly - so perhaps it's time to spruce up the house over winter.

The findings from the pioneering research project were aired on BBC's Trust Me, I'm A Doctor, this Wednesday.